(a) Field of the Invention
The present invention relates to a process for manufacturing a semiconductor device, and more specifically, it relates to a process for manufacturing a semiconductor device in which a via hole formed in an interlayered insulating film is filled with a wiring metal.
(b) Description of the Related Art
With the high integration of semiconductor devices, a transverse size of elements and wires is being miniaturized more and more. However, the miniaturization of a vertical size, i.e., the miniaturization of the vertical size of interlayered insulating films and wires does not progress because of the increase in a wire capacity, a poor dielectric strength of the interlayered insulating films and the like. Therefore, there increases a ratio (aspect ratio) of the diameter to the depth of a via hole which is provided through the interlayered insulating film in order to connect a wire to an element or a lower wire or the like. Therefore, step coverage properties in the via hole of a metal film such as an aluminum alloy formed by a usual sputtering process are poor, and it is difficult to meet required characteristics.
In the case that this metal film is an aluminum-based alloy film comprising aluminum or an aluminum alloy, there have been suggested various methods for improving the step coverage properties by the utilization of a fact that this metal film has a low melting point. One of these methods is a bias sputtering process in which a DC or a RF bias is applied to a semiconductor substrate during the sputtering of the aluminum alloy film. In this method, the kinetic energy of argon ions struck on the semiconductor substrate is converted into heat energy, so that the aluminum alloy film is molten, whereby the step coverage properties of this aluminum alloy film can be improved. In this method, however, the element is damaged and the aluminum alloy film is degraded by the argon ions struck on the surface of the semiconductor substrate, and these problems have not been solved yet. For this reason, the bias sputtering process has not been put to practical use.
In addition to the bias sputtering process, another method has been suggested in which the aluminum alloy film is formed by the usual sputtering process, and laser beams or electron beams are then struck on the aluminum alloy film to melt the same, thereby improving the step coverage properties of the aluminum alloy film. For example, a method of melting the aluminum alloy film by the electron beams is disclosed in Japanese Patent Application Laid-open No. 188267/1991.
This method will be described with reference to drawings. FIGS. 3a-3c are sectional views illustrating the main process of this method. As shown in FIG. 3(a), a contact hole for connecting to an element is formed at a desired position of an interlayered insulating film 22 comprising a PSG film (phosphosilicate glass) on a silicon substrate 21 provided with the element, and a titanium film 23 having a thickness of about 50 to 100 nm is then formed so as to cover the interlayered insulating film 22 or the contact hole by a sputtering process.
Next, as shown in FIG. 3(b), an aluminum alloy film 24 having a thickness of 0.4 to 1.5 .mu.m is formed by a usual sputtering process. Afterward, as shown in FIG. 3(c), the silicon substrate is maintained at a temperature of 400.degree. C., and the aluminum alloy film 24 is then molten by the irradiation of electron beams to fill the contact hole with the molten aluminum alloy. In this case, the energy of the electron beams to be irradiated is about 100 .mu.sec at 1 to 3 keV and at a current of 15 to 35 A. These serial processing steps are preferably carried out in one vacuum atmosphere, and in particular, the formation of the titanium film 23 and the aluminum alloy film 24 is done in the one vacuum atmosphere so that any oxide film may not be interposed therebetween, with the result that the wetting property of the aluminum alloy film is good and the filling of the contact hole is easy.
Furthermore, another method has also been suggested in which the aluminum alloy film is molten only by the heating of the substrate without using any energy beams such as the laser beams or the electron beams. For example, as reported in Proceeding of 1991 VMIC Conference, pp. 326-328, the aluminum alloy film is formed at a low temperature of 100.degree. C. or less by the usual sputtering process, and the semiconductor substrate is then heated at 400.degree. to 550.degree. C. in one vacuum atmosphere to melt the aluminum alloy film.
Alternatively, as reported in Proceeding of 1992 VMIC Conference, pp. 219-225, a barrier metal film is formed, and the aluminum alloy film is then formed, while the semiconductor substrate is heated at about 500.degree. C., to melt the aluminum alloy film. As the barrier metal film, a titanium film, a TiON film and a TiN film have been tried, but the titanium film is most excellent in wetting to the aluminum alloy film and is easy to fill the contact hole.
In a conventional method for manufacturing the semiconductor device in which this silicon substrate is heated to a high temperature and the aluminum alloy film is then formed, the titanium film is as thick as about 100 nm, and so a TiAlx alloy layer which is formed by the reaction of the aluminum alloy film and the titanium film is also thick. This TiAlx alloy layer is not uniformly formed, and as shown in FIG. 4, its film thickness is nonuniform and wavy. The thicker the formed TiAlx layer is, the wavier the surface of the TiAlx alloy layer is. Hence, the aluminum alloy film on the TiAlx alloy layer is affected by this TiAlx alloy layer, and thus it also becomes wavy. The surface state of the aluminum alloy film is particularly bad at the step portion.
If the surface state of the aluminum alloy film is bad, it is difficult to achieve precise positioning to the contact hole and a through hole, when a photoresist film formed on the aluminum alloy film is patterned to a desired shape of an aluminum wire. In an extreme case, the positioning is quite impossible.
Furthermore, in the case that a wire is formed from the layered structure of the titanium film and the aluminum alloy film, the thus formed wire is then covered with an interlayered insulating film or a passivation film, followed by a heat treatment at about 400.degree. C. At this time, the aluminum alloy film is partially lost by the stress of the interlayered insulating film or the passivation film. That is to say, the so-called stress migration easily occurs. In particular, the thicker the titanium film is, the more easily the stress migration occurs and the more easily the wire breaks inconveniently.
If titanium gets into the aluminum alloy film, the resistance of the aluminum alloy film increases. When the titanium film is thick, a large amount of titanium gets into the aluminum alloy film, and the TiAlx alloy layer in the contact hole is also thickly formed, so that there is a problem that the wire resistance increases.
As described above, in the method for preparing this kind of semiconductor which comprises forming the aluminum alloy film at the high temperature to fill the via hole therewith, the titanium film having a thickness of about 100 nm is used, but the reason why such a thick titanium film is used will be described as follows.
When the aluminum alloy film is formed at a high temperature, the aluminum alloy is easily flowable, and so, before the melting of the formed aluminum alloy film, the aluminum alloy film is easily separated at the contact hole into an upper portion and a bottom portion, as shown in FIG. 5(a). If the titanium film is thin and it does not have a sufficient film thickness on the side wall of the hole, the flow of the aluminum alloy film does not reach the bottom of the hole owing to the discontinuous aluminum alloy film, even when the aluminum alloy film is molten. In consequence, disconnection occurs, as shown in FIG. 5(b). However, if the titanium film is thick and has the sufficient film thickness on the side wall of the hole, titanium is replaced with aluminum and the aluminum alloy film on the upper portion of the hole flows into the hole, so that the surface of the aluminum alloy film can be flattened. Therefore, the film thickness necessary for the titanium film is at least about 50 to 100 nm, depending upon the aspect ratio of the hole.
As reported in Proceedings of 1992 VMIC Conference, pp. 219-225 mentioned above, oxygen in the interlayered insulating film oxidizes the titanium film to deteriorate filling properties. Therefore, when the titanium film is directly formed on the interlayered insulating film, the titanium film having a thickness of about 100 nm or more is necessary so as to accept the slight oxidation of the titanium film.
In the method in which after the formation of the aluminum alloy film, the aluminum alloy film is molten by the irradiation of the laser beams or the electron beams, the surface of the aluminum alloy film is oxidized, and so the temperature of the aluminum alloy film reaches a high level of 550.degree. C. or more. If the temperature of the aluminum alloy film is so high and the titanium film is thin, the aluminum alloy film tends to aggregate in the form of grains. In order to prevent this tendency, the titanium film is also required to have a film thickness of about 50 to 100 nm.
In the method in which after the formation of the aluminum alloy film at a low temperature by the usual sputtering, the silicon substrate is heated to melt the aluminum alloy film, there is not any report that the titanium film is used as an under film for the aluminum alloy film, but the film thickness of about 50 to 100 nm is often used, as in the other methods.
In particular, when a base pressure in a process chamber in which the aluminum alloy film is formed and the substrate is heated is higher than 10.sup.-7 Torr, the aluminum alloy film is oxidized, so that it scarcely flow. In order to make the aluminum alloy film flowable, it is necessary to heat the aluminum alloy film to a high temperature, but if the aluminum alloy film is heated to the high temperature and the thin titanium film is used, the aluminum alloy is inconveniently liable to become grains. In consequence, the titanium film is still required to possess a film thickness of about 50 to 100 nm.
Instead of directly forming the titanium film on the interlayered insulating film, a TiN film may be formed as a barrier metal for preventing the interdiffusion of aluminum, silicon and the like, and the titanium film may be then formed thereon. In this case, the oxidation of the titanium film does not have to be considered and unreacted titanium is present in the TiN film, and so the titanium film can be thinned. In general, a thickness of about 30 to 50 nm is often used.
Even if the titanium film is thinned to about 30 to 50 nm, problems such as the bad state on the surface of the aluminum alloy film and the deterioration of a stress migration resistance cannot be solved.
Therefore, it is usually unavoidable to set the film thickness of the titanium film to about 50 to 100 nm.